i considered f(x) = y for simplicity.
i'll write out steps so that you can understand.
y= 7x-13
y+13 = 7x
x = (y+13)/7
now to find inverse function just replace x and y
y= (x+13)/7
so your inverse function is
\[f^{-1}(x)=\dfrac{x+13}{7}\]
got this ?